PORTSMOUTH — The owners of Redhook Ale Brewery hired an independent investigator to probe the cause of a keg explosion that fatally wounded an employee in April.

Craft Brew Alliance, Inc., Redhook's parent company, provided new information about the industrial accident last week in a communication sent to members of the brewing industry.

In a June 1 message, CEO Terry Michaelson wrote CBA has not yet determined what caused the plastic keg to explode, killing an employee who was cleaning the vessel with compressed air.

With the source of the keg failure still a mystery, CBA hired an independent investigator to assess the keg and cleaning equipment involved in the accident, according to Michaelson. Federal inspectors from the Occupational Safety and Health Administration are still investigating the incident.

One factor complicating the investigation is that there were no witnesses to the 7 a.m. explosion, according to Michaelson. He wrote the company might never be able to ascertain the exact cause.

In the wake of the April 24 blast, several brewers around New England were perplexed by the accident, which stands out as one of the only instances of a work-related keg explosion most beer industry veterans can recall.

The incident raises eyebrows in the brewing community because Redhook officials said it involved a plastic keg, rather than a traditional stainless steel keg — the industry standard for several decades.

"Although we are still awaiting conclusions from OSHA, we recognize the importance of sharing information about the accident with the brewing industry in hopes of preventing similar accidents," Michaelson wrote.

Based on a preliminary investigation, Redhook believes the worker was in the process of emptying the keg in order to stack it on a pallet when it blew apart.

"We empty the kegs in order to increase their stability when stacked on a pallet before being returned to their respective owners," Michaelson wrote.

He continued, "For years, this process for emptying kegs has been performed without incident in a substantially identical manner at the Redhook Brewery and all of our other breweries."

When the keg ruptured, it split into two pieces, bursting at the middle seam where the two halves of the keg were joined, according to Michaelson.

The keg involved in the accident was not owned by Redhook or any other brewery under the Craft Brew Alliance umbrella, the company has said. The keg was delivered to the brewery in error as part of a delivery of empty keg returns.

The keg wasn't clearly marked with the manufacturer's name, but CBA is in contact with a manufacturer believed to have produced the keg.

"In the meantime, CBA remains committed to safety at all four of our brewery locations," Michaelson wrote. "Our safety manager and local safety coordinators continue to hold monthly on-site meetings with each of our brewing and operations teams. Ongoing training remains a top priority for our team."

CBA is also planning to make investments in safety equipment this year and hire outside safety consultants, according to Michaelson's message.

"We will continue to work toward improving safety at all brewery locations to ensure that we provide the safest work environment possible for all of our employees," he wrote.

OSHA spokesman Ted Fitzgerald said an investigation was launched into the accident, but declined to discuss the inquiry while the process is ongoing.

The Redhook Ale Brewery in Portsmouth has never been cited for a workplace safety violation, according to information available from OSHA.

Michaelson concluded the June 1 message by thanking members of the craft beer and brewing communities for their support and patience in the wake of the tragedy.

The company is tentatively planning to brew a memorial beer to celebrate the life of Ben Harris, the Pennsylvania native killed in the accident.

"More than anything, we hope that the information we're able to share in the wake of this tragic accident contributes to greater awareness about safety and to safer work environments throughout the industry."